The paid version will delete your email address if you don't keep up with payments.

I emailed them a little over a year ago about this because the terms of service were unclear. I was told by support that my main address wouldn't be deleted, but I would be moved down to the free tier in every way -- so I would lose my extra addresses and aliases, as well as extra storage space. I was also told that there was a 30-day grace period in case my renewal payment didn't go through for some reason.